The NewSoftSerial library is declaring interrupt service routines for the pin change interrupts even if it doesn't use them. Here's a quick explanation of how the pin change interrupts work.
Almost all of the IOs on the ATMega328P can be used as an interrupt, but only to detect if there's a level change (high to low, low to high). This is in addition to the two dedicated external interrupt pins on the chip. If you look at the datasheet, you'll see that the pin names have designations like PCINT0 to PCINT23. The ATMega328 groups 8 pin change interrupts together into one interrupt vector so in total, there are three dedicated pin change interrupt vectors called PCINT0_vect, PCINT1_vect, and PCINT2_vect.
PCINT0_vect is the vector that handles any interrupt from physical pins PCINT0 to PCINT7, PCINT1_vect handles interrupts from physical pins PCINT8 to PCINT15, etc...
The chibiArduino stack uses physical pin PCINT6 (XTAL1/PB6) on the ATMega328. That means that its interrupt vector gets mapped to PCINT0_vect. In the NewSoftSerial Library, it declares interrupt service routines for all three pin change interrupt vectors PCINT0_vect, PCINT1_vect, and PCINT2_vect, even if they are not used. Hence, you can comment out the service routine in the NewSoftSerial library if you're not using the associated pins for serial data.
The chibiArduino stack requires the PCINT0_vect to operate. If you comment out (disable) the PCINT0_vect from the NewSoftSerial library, you will not be able to use Arduino pins 8-13 for software serial data. Actually pins 11 to 13 are used for the SPI so basically, you'd be restricted from using digital pins 8 to 10. If you're okay with this, then its possible to use chibiArduino and NewSoftSerial together. Here is the modification I made on the NewSoftSerial library so that the two stacks can compile when used together.
NewSoftSerial.cpp, approximately Line 315